I should add: The app I want to use is Xfinity Stream - for users of Comcast’s cable service. This Android app works fine on tablets and other non-HDMI devices, so I believe it checks for an HDMI connection.

There is some way to do this; I’m not a programmer but I got across st least two devices both from China which have no problem with Xfinity Stream latest version when connected to TV via HDMI. They both running Android 4.4. I wish someone will modify Xfinity APK and offer it for sale. One will think that it should be easy task for software engineers.